Dynamic Equilibrium Notes

Dynamic Equilibrium

Introduction

• Dynamic equilibrium is a state where a system has reached its optimal condition.
• Analogized to a shopping experience to illustrate the concept.

Shopping Analogy

• Scenario: A store opening with a deal, many customers (reactants) outside, and an empty store (no products initially).
• Initial State:
  • Outside the store: 100 people (reactants).
  • Inside the store: None.
• As Doors Open: People rush in to make purchases.
• Equilibrium:
  • People enter and exit the store.
  • Dynamic equilibrium is reached when the number of people entering equals the number of people leaving per unit time.
  • The total number of people inside the store remains relatively constant, although the individuals change.

Chemistry Perspective: Requirements for Dynamic Equilibrium

1. Reversible Reaction:

   • The reaction must be able to proceed in both forward and reverse directions.
   • Some reactions go to completion, where reactants almost entirely convert to products.
   • More commonly, reactions result in a mix of reactants and products.

2. Equal Forward and Reverse Reaction Rates:

   • The rate at which reactants convert to products (forward reaction) must equal the rate at which products convert back to reactants (reverse reaction).
   • Example: If six molecules of reactants convert to products per second, then six molecules of products must convert back to reactants per second.

3. Ongoing Reaction:

   • The reaction continues even when there is no observable change in concentrations.
   • At equilibrium, the rates of the forward and reverse reactions are equal ($rate<em>{forward} = rate</em>{reverse}$).

Concentration vs. Time Graph

• Reactants and Products:
  • Reactants are converted into products.
  • Products are converted back into reactants.
• Visual Representation:
  • $R \rightleftharpoons P$
• Equilibrium:
  • Concentrations of reactants and products remain constant.
  • The reaction is still occurring at the molecular level.

Law of Mass Action

• General Reaction: $A \rightleftharpoons B$
• Equilibrium Constant (K):
  • Defines the ratio of products to reactants at equilibrium.
  • $K = \frac{[Products]}{[Reactants]}$
  • For the reaction $A \rightleftharpoons B$, $K = \frac{[B]}{[A]}$
• Reactant-Favored vs. Product-Favored:
  • If K < 1: Reactant-favored (more reactants than products at equilibrium).
  • If K > 1: Product-favored (more products than reactants at equilibrium).

Equilibrium Constants and Solubility

• Solubility:
  • High solubility: Products (ions) are favored; K > 1.
  • Low solubility: Reactants are favored; K < 1.
• Example: Calcium Carbonate (Chalk/Limestone)
  • $CaCO<em>3(s) \rightleftharpoons Ca^{2+}(aq) + CO</em>3^{2-}(aq)$
  • Low solubility.
  • K value: $5.0 \times 10^{-9}$ (very small, indicating reactant-favored conditions).
  • This means very little calcium carbonate will dissolve into its ions.

Manipulating Reactions

• Methods:
  • Changing temperature.
  • Changing concentrations of reactants.
  • Changing volume and/or pressure.
• Le Chatelier's Principle:
  • Used to predict how changes in conditions affect equilibrium.